Softening agent for rubber and resulatant rubber composition



Patented Jan. 15, 1952 SOFTENING AGENT FOR RUBBER AND RESULTANT RUBBERCOMPOSITION 4 Frank M. McMillan and De Loss E. Winkler,

Berkeley, Calif assignors to Shell Development Company, San Francisco,Calif., a corporation of Delaware No Drawing. Application June 25, 1949,Serial No. 101,503

9 Claims. (Cl. 260-761) This invention provides novel softening ingredients for use in the working of rubbers, and it is alsodirected to theresulting rubber compositions.

As employed, herein, the term rubber embraces both natural rubber, inall its various raw and reclaimed forms, as well as the varioussynthetic rubbers, i. e. rubbery polymers, of the type which may bevulcanized with sulfur. Representative synthetic rubbery polymers ofthis variety are the homopolymerization products of butadiene and thoseof its homologues and derivatives, as, for example, methyl butadiene,di-

methyl butadiene, pentadiene, and chloroprene ..(neoprene syntheticrubber), as well as copoly- .well as vinyl esters and variousunsaturated aldehydes, ketones and ethers, e. g. acrolein, methylisopropenyl ketone, and vinyl ethyl ether.

.The foregoing rubber materials, including both natural rubber as wellas the various synthetic rubbers, may be defined as rubbery polymers ofat least one compound selected from the group consisting of theconjugated diolefins and chloroprene.

Rubber compositions of a type suitable to be vulcanized are prepared byblending with the rubber stock a number of other ingredients.

However, the physical condition of said stocks is usually such that thiscompounding may be effected only as the rubber is masticated on a .millor in a mixer until it becomes plastic This mastication process isnormally productive of large amounts of heat, with the result that there.is-danger of prematurely vulcanizing the composition. However, theamounts of heat generated in this fashion may be considerably reduced byadding to the rubber being worked one or more oils, fats, waxes, tars,or the like which have an aflinity for rubber and act to swell andsoften the same, thereby accelerating the mechanical action of themixer.

The effectiveness of the foregoing softening agents varies greatly fromone material to another, some acting principally to reduce the amount ofheat generated during the mixing.

process, while others impart tackiness to the mixture and therebyincrease its homogeneity and overall workability. Still others haveimportant scorch retarding properties and thereby reduce the tendency ofthe rubber stock to become vulcanized at the temperatures normallyencountered during working on the mill without aifecting the cure rateat the higher temperatures employed in the subsequent vulcanizationsteps. Some few softening agents possess more than one of thesequalities in some measure, but only an extremely limited number of suchcompounds are sufiiciently well rounded as to be regarded as all-purposesofteners. One such material is pine tar, a composition which findsparticular utility in connection with various tire stocks as well aswith rubber compositions used for other purposes, and it is an object ofthis invention to provide an economical material of petroleum originwhich will have all the desirable attributes of pine tar and may beemployed as a substitute therefore either in whole or in part.

It is our discovery that a composition made up of from about 15 tonaphthenic acids and at least 15% petroleum residues of hereinafterdefined composition, with the balance of the composition beingessentially comprised of a neutral oil, represents an excellent softenerfor use in both natural as well as synthetic rubbers. This compositionnot only has the ability to soften the mass and supply the desiredcondition of tackiness, but it also decreases the scorchiness of therubber and permits the effective com-4 pounding thereof at normaloperating temperatures. The resulting composition has a high Mooneyscorch time coupled with a good elastic recovery or nerve. Thevulcanizate prepared from rubber compositions incorporating the softenerof this invention is characterized by a normal modulus of elongation anda relative high tear strength. In short, both the unvulcanized as wellas vulcanized compositions prepared with the naphthenic acid-petroleumresidue softener of this invention possess all the desirable attributesof those compounded with pine tar and in some respects are even superiorto the latter.

The naphthenic acids employed in this invention are compounds of thetype derived on the acidification of alkali extract of crude petroleumoils or their distillates. The mixture of acids obtained in this fashionmay either be used as such, in which case it may normally contain fromabout 20 to 40% by weight of a neutral oil, or it may first be subjectedto a preliminary refining to 12 carbon atoms in the molecule, whereasthe higher boiling acids, which usuallyare-a-lsool the monocarboxylicvariety are polycyclic in character and contain up to 30carbon atoms inthe molecule. The naphthenic Tacid "mixtures preferably employed informing theriibber softeners of this invention are those having anaverage acid number of from about 'l00 to;250 as measured on an oil-freebasis. As employed herein, the acid number is expressed in terms of thenumber of milligrams of potassium hydroxide required to neutralize onegram of the naphthenic acid.

I The petroleum residues which are "combined with the naphthenic' acids"to form thesofteners of this invention are highfmolecularweightfaromaticand naphthenic hydrocarbons whichmay .or may not 'containfsulfur andwhich are "obtainedias residues and extracts in processe involvf'ing'removal'of light endsand' the various lubricating oil fractions from'crudeoils. mended among'thesa'id products are the yarious resid'ualasphalts, either per se or in the-form of liquid mulsionsor solutionswith an oil distillate, as lwellfas the liquid extracts obtained bytreating them with agents-of the type ofsuli'ur'dioxide,

'furfural, phenol, propane, cre'sol or the like. These petroleumresidues vary in 'viscosity-fromabout 10 cs. at 210 F. up to moreo-rless solid asphaltic compositions softening at about 100 to 200 F.,and

in specific gravity (di fromabout 0.9: to 1.05. L40

Their initialboiling pointis at least 300 F. 'at 1 mm. Hg. Because ofthe predominantly aromatic character of the herein definedre'sidues,thesame may otherwise be describedas"aromatic-petroleum derivatives, i.e., as aromatic fractions derived from'naturally occurring, as well ascracked-petroleum stocks.

I In compounding the softeners of the invention, these various petroleumresidues may be employed either singlyor in combination. Thus, excellent'resultshave been obtained by the use inrubbers of a-mixturemade up ofunrefined naphthenic acids and an asphaltic type residue andoptionally-with a so-called neutral, or-volatile petroleum oil. Again,good results have also been obtained by employing the-naphthenio acid incombination-with asolution-of an asphaltic residue in a .Duosol,Edleanea -or furfuralextract. Likewise, it -is also possible to omit theasphalticf component altogether and to obtain a goodrubben softeningagent -by employing as the petroleum residue one-or; more of the saidDuosol, Emee'n eau .or furf-ural extracts in combination withthe{naphtheni acid mixture. However, -it isj'preferred not toomitthe-asphaltic residue" altogether "from the softeningcomposition. vv

The naphthenic acid and petroleum residue components of the softener maybe added to'the -rubber=either separately or in the form of arpreviously prepared composition. Ineithercase, M

however, it is importantthat the overall naphthenic acid content of thesoftener not'exce'ed about 60%, for with larger amounts there ensues anundesirable increase in the modulu of'the *final'yulcanizate, e., thevulcanized articles become stiffer than wouldotherwisebethe case and agreater force is required to stretch them in any predetermined amount.On the other hand, the naphthenic acid content of the softener shouldnot fall below about 15% for there then ensues" a rapidincreaseinthescorchability of the stock together with an impairment of its 7tackiness and overall workability. A preferred "proportional range forthe softener ingredients is *from 20-to;40%: by weight of naphthenicacids and 'from'60 to by weight of petroleum residue together with anyneutral petroleum oil (i. e.

" any volatilefor'distillable petroleum oil) present. In I the "latterconnection, it is to be observed -that 'theneutral petroleum oil, Whichfrequently acts as a base or solvent for the naphthenicacidj asphalt, orother petroleum residue, "nriaybe present in an amount up to about 50%of the'overall weight of the soften-er without deleterious effects,provided that the composition shall always contain at least about 15% ofthe petroleum'residue, in addition to its kion'te'nt of naphthenic acid.'fIhe'various percentagesexpressed herein for the naphthenic acid'a'ndpetroleum residuef'co m'ponents of the softeners are on an oil-free?basis.

'tral petroleum oil present'retains the high Mooney scorch time"and'most pr the tack or "a stock softened only Iwith naphthe nic acid I(or "with naphthenic "acid plus neutral"oil), 'while'at the same timethe modulus of the vulcanizate is reduced to 'a normal level' ascompared with the abnormally high value which prevails when'thepetroleum residue is omitted. H

In working up the rubber with the softener, the components of the latterare 'added'to the rubber either separately or in the form of previouslyprepared liquids or semi-liquids, followed by thorough mixing toattain'a mass of the desired plasticity and tackiness, after which theother compounding ingredients are "normally worked into the product. Theresulting mixture is then subjected to the desired shaping andvulcanizing operations. The amount of softener to'be employed issusceptible of considerable variation, de-

pending both on the nature of this additive'as well'as on that of therubber "and other compounding ingredients used. However, gcodresults areobtained by uslngfrom about 2 to 20 parts of the softeningcompositionfor each parts of rubber, and preferably there areemployedfrom 5 to 10 parts of the softener for each 100' parts oftherubber.

Subject to the proportional limits described 1' above, any one or moreof the various; naphthenic 'acid mixtures, and petroleum residues lrnownin 'theart may beincorporat'ed to advantage with any organic, rubberypolymer "ca'pable'of bein v'ulcanizedwith sulfur. A waspointed outaboye,'this class 'of"'compounds includeshot only the various natural rubbers,but also the'synthetic rubbers prepared in whole or in part from'conmore of the above-identified softeners with one 'or more rubbers, orrubbery polymersmf the type vulcanizable with sulfur, and with orwithout other rubber compounding ingredients as noted or other liquid,as in the case of latex naturalrubber. The softeningbompositions of thisinvention may be added to such emulsions eitherper se or in the form ofother emulsions, after which the mass is precipitated and worked up in amanner known in the art.

In addition to the softening ingredient, the vulcanizable compositionnormally contains other additives, of which one is necessarily thevulcanizing agent. Suitable agents of this character are sulfur, sulfurchloride, sulfur thiocyanate, thiuram polysulfides and other organic orinorganic polysulfides, organic and inorganic peroxides,halogen-containing compounds and nitrogen-containing compounds such asthe nitrobenzenes.

vulcanization accelerators and accelerator-activators may be present,particularly when sulfurcontaining compounds are ingredients of thecompositions. Illustrative examples of accelerators are tetramethylthiuram disulfide, zinc dibutyl dithiocarbamate, tetramethyl thi-urammonosulfide, dipentamethylene-thiuram tetrasulfide, mercapto benzothiazole, hexamethylene-tetramine, aldehyde-ammonia, diphenylguanidine,diphenylthiourea, benzo thiazyl disulfide, pip-eridiniumpentamethylene-dithiocarbamate di o-tolylguanidine and leaddimethyldithiocarbamate. The metal oxides such as zinc oxide and leadoxides are frequently added as accelerator-activators.

' While the softening agents of the present invent exhibit aplasticizing as Well as softening and tackifying action, in some casesit may also be desirable to use additional plasticizers of which dibutylphthalate, dioctyl phthalate, tricresyl phosphate, triacetin, tetralin,cumar resins, soft factice, wool gloss, stearic acid, and lauric acidare examples. Pine tar itself may comprise a portion up to 50% of thesoftening agent composition.

Having compounded the rubber by mixing therewith the above-describedmaterials, the mix is vulcanized by treatment at elevated temperatures,preferably above 100 C., for periods varying from several minutes to oneor more hours. Preferred vulcanizing treatments are those conducted attemperatures of from 125 to 175 C. for from to 45 minutes.

Compositions can be produced in accordance with the invention which arecapable of use for substantially all of the purposes for which otherrubber-like compositions are employed. Exam- Dles which come readily tomind are automobile tires and tubes, floor tiling's, balloon coverings,umbrellas, raincoats, table covers, shower curtains, garment bags,electrical insulation, friction tape, hose for the handling of aqueoussubstances and of petroleum products and paints, lining and exteriorcoating in self-sealing gasoline tanks, gaskets, belts, shoe soles andheels, aprons, gloves, sporting goods, such as rubberballs, bathingsuits and caps, and drug sundries.

The present invention is illustrated in various of its specificembodiments by Examples I through IV below, in each of which there wasused a natural rubber tire carcass stock which had been formed on atwo-roll rubber mill by blending 10 parts of a softener'with a rubbercomposition made up of 80 parts smoked sheet, 10 parts rolled browncrepe, parts whole tire reclaim, and parts of carbon black, andthereafter milling into the composition 1 part stearic acid, 5 partszinc oxide, 0.8 part mercaptobenzothiazole, 2.75 parts sulfur and 0.5part Santoflex B, a reaction product of acetone and p-aminodiphenyl. Onelot of each of these stocks was then cured for 20 minutes at theconventional vulcanizing temperature of 145 C. whereas another lotthereof was cured for the same length of time at the abnormally lowvulcanizing temperature of 126 0. Samples developing a comparativelyhigh tensile strength at such a low temperature, as this are having anundulyhigh scorchiness.

EXAMPLE I The softening agent used in preparing the rubregarded as berstock described in the preceding paragraph Table I which follows ExampleIV, this composition had good tackiness properties and its Mooney scorchtime at 121 C. (14 minutes) was ood. As further evidence of a lack ofscorchiness in the product, the sample vulcanized at 126 C. had therelatively low tensile strength of 1720 p. s. i.

EXAMPLE II In this case, the rubber stock described above was compoundedwith 10 parts of a softener made up entirely of a Duosol extractobtained by treating an oil distillate with phenol and propane and thendistilling off the phenol from the extract so obtained. The physicalproperties of the Duosol extract here employed as a rubber softener wereas follows:

Specific gravity D4 1.02 Viscosity, 210 F. (cs) -1 32 Flash point, P.475 Initial boiling point (1 mm. Hg) "F 444 As will be seen from thedata of Table I, the stock prepared with this softener was highlyscorchy, it having the low Mooney scorch type of but 10.5 minutes anddeveloping the high tensile strength of 2470 p. s. i. when cured at 126C.

EXAMPLE III In this example, the softener was made up of 5 parts of theDuosol extract described in Exam ple II above and 5 parts of anaphthenic acid mixture derived on acidifying an alkali extract ofpetroleum and containing 27.6% neutral oil, the naphthenic acid mixturehaving an acid number of 146. On an acid-free basis, the acid number ofthe naphthenic'acid would be 202. The overall naphthenic acid content ofthe softener on an oil-free basis was 36.2%. In preparing the rubberstock, the Duosol extract was added first and then the naphthenic acidwas worked into the mixture; As shown in Table I, this stock hadexcellent anti-scorch properties, its Mooney scorch time being 16minutes and the sample having a low tensile strength when cured at 126C. In this respect the properties of the sample were much superior tothose of the stock described in Example II, and they were even superiorto those of the rubber compounded with pine tar, the commerciallyaccepted softener. At the same time, the present stock had good tack andworked up and handled very well.

EXAMPLE IV This stock was prepared in the same manner as that describedin Example II except that'here the amazes e r be in Exam e III and in to asphalt petroleum residue. The latter residue, which contained 30% ofaneutral oil which acted as a solvent for the asphalt, had the followingphysical properties:

Specific gravity D4 0.98 Viscosity, 100 F. (cs.') 3540 Flash point, F450 Initial boiling point (1 mm. Hg), "F 302 As will be seen from thedata given in Table 2, this material had excellent all-around softenerTable 1.Efiect of naphthem'c acid-petroleum residue softeners onphysical properties of natural rubber tire carcass stock Example IIIfgfig g g' q. a Example g g i 'gz tgg themca'cm, Product described in I,100% f 13 neutral 6.9% neutral Tar extract oil 26nd 50% 632 37 Duosolextr. tmot A. Unvulcanized .Stock:

ac excellent excellent excellent excellent ficorchtime (Mooney) min. at121 O 14 10. l0 13 B. Vulcanizatc cured m at 145 0.:

Tensile strength, p. s. i. -.-,-I----- 2, 445 2, 255 2, 770 2, 450Elongation at break, per cent 715 790 750 705 Stress at 300% elongation,p. s. i. (modulus) 200 200 290 210 C. Vulcanizate cured 20min.at12d,O.-Tensile strength, 1, 720 2, 470 1,670 2,150

In Examples V through XII to follow, which are also illustrative of thepresent invention, there was employed'a natural rubber tire tread stockwhichhad been formed on a two-roll rubber mill by blending" 10 parts ofasoftener with a rubber composition made up of 100 parts smoked sheetadmixed with 50 parts of carbon black,'and thereafter milling into theresulting plastic mass 1 part mercaptobenzothiazole, 1 partphenylbeta-naphthylamine, 5 parts zinc oxide, 4 parts stearic acid and 3parts sulfur. One lot of each of these stocks was then cured for minutesat the conventional vulcanizing temperature at 145? C. whereas anotherlot was cured for the same length of time at the relativel lowvulcanizing temperature of 126? C. As was the case with the carcassstocks described in Examples I through IV above, those vulcanizateswhich developed a relatively high tensile strength when vulcanized at126 C. are regarded as being unduly scorchy.

EXAMPLE v The softening agent employed in this case was '10 parts ofpine tar, the resulting softened rubber stock having good tackiness anda low Mooney scorch time. As further evidence of a lack of scorchinessinthe product, the sample vulcanized at 126 C. had a relatively lowtensile strength.

In thisexample the rubber was compounded with 5 parts of the naphthenicacid composition characteristics, the stock having a high Mooney scorchtime and exhibiting good tack and workability, and the vulcanizatehaving a normal m ul s III The rubber stock of this example was preparedusing 2.5 parts of the naphthenic acid composition describedin ExampleIII and 7.5 parts of the asphalt solution described in Example VII, theoverall content of naphtheni'c acid in the softener thus being 18.1% asmeasured on an oil-free basis. The action of this softener was generallythe same as that of the 50/50 naphthenic acidasph'alt softener ofExample VII, though here there was some evidence of an increase inscorchiness in the stock.

EXAMPLE IX The softener employed in preparing the stock of this examplerepresented 10 parts of the asphalt composition described'in ExampleVII. As will be observed from the data presented in Table 2 the physicalproperties of this stock were generally unsatisfactory, its tackinessand workability not being up to commercial requirements and thescorchiness oi the stock being relatively high.

EXAMPLE X This rubber stock was prepared using 10 parts of a 60%solution in 'a neutral oil base of an asphalt petroleum residue, thesolution having the following physical properties:

Specific gravity D43 1 Viscosity, F., (cs.) 12,000 Viscosity, 210 F.(cs.) 58 Flash point, F 405 Initial boiling point (1 mm. Hg), F 340 Aswas the case with the stock prepared in the preceding example. thismaterial also had poor tackiness. Further, as manifested both by a lowMooney scorch time and the extraordinarily high tensile strength (1825p. s. i.) of the vulcanizate prepared at 126 'C., the stock was veryscorchy.

9 EXAMPLE XI This stock was prepared using 7.5 parts of the asphalticsolution described in the preceding example and 2.5 parts of thenaphthenic C0111 position described in Example 111, the softener havingan overall naphthenic acid content 18.1% as measured on an oil-freebasis. As will be seen from the data of Table 2, the introduction ofeven this relatively small amount of naph thenic acid served to restorea normal degree of tackiness to the mixture while at the same timegreatly improving its scorchiness characteristics.

EXAMPLE XII This stock was prepared in the same fashion as thatdescribed in the preceding example except here there were employed partsof the naphthenic acid solution and 5 parts of the asphalt solution, thesoftener thus containing 36.2 naphthenic acid as determined on anoil-free basis. The data presented in Table 2 shows the stock preparedwith this softener to be satisfactory in every particular, it havinggood tack and workability coupled with a very low scorchiness.

from about 20 to 40% naphthenic acids, at least of an aromatic petroleumderivative, and from 0 to 50% of a neutral petroleum oil, said aromaticpetroleum derivative having a viscosity of at least 10 cs. at 210 F.,softening below 200 R, and having a specific gravity ((14 of from 0.9 to1.05.

6. A softened rubber composition made up in the ratio of 100 partsrubber from 2 to 20 parts of a softener composition comprising fromabout 15 to 60% naphthenic acids, at least 15% of an aromatic petroleumderivative, and from 0 to 50 of a neutral petroleum oil, said aromaticpetroleum derivative having a viscosity of at least 10 cs. at 210 F.,softening below 200 F., and having a specific gravity ((14 of from 0.9to 1.05.

7. The composition of claim 6 wherein the petroleum residue is anasphaltic material.

8. The composition of claim 6 wherein the petroleum residue is anextract obtained by treating petroleum with propane and phenol.

9. The composition of claim 6 wherein the petroleum residue is anEdleaneau extract.

FRANK M. McMILLAN. DE LOSS E. WINKLER.

Table 2.Efiect of naphthem'c acid-petroleum residue softeners inphysical properties of natural rubber tire tread stock Example ExampleExampie Example Example VII, 36.2% VIII, 18.1% Example Example XI 18.1%XII, 36.2% Example VI, 72.4% naphthenic na hthenic IX, 70% naphtbenicnaphthenic Product described in. V, 100% naohthenic acid acid, 52.5% ashalt, as halt, acid, 45% acid, 30%

Pine tar acid, 27.6% asphalt, asphalt, 30% neuneuasphalt, asphalt,

neutral oil 28.9% neu- 18.1% new trial oil tral oil 36.9% neu- 83.8%neutral oil tral oil tral oil tral oil A. Unvulvanized stock:

Tack good good good good only fair only lair good good Scorch time(Mooney) min. at 121 C 48 48. 5 54 52 44 61 B. Vulcanizate cured 30 min.at 145 0.:

Tensile strength, p. s. i 3, 500 8, 690 3, 835 3, 970 3, 720 3, 745 3,885 3, 785 Elongation at break, per cent 650 550 565 670 690 680 665 665Stress at 300% elongation. p. s. i.

(modulus) 940 1, 420 1, 010 1, 020 870 1, 050 1, 080 1, 060 O.Vulcanizate cured 30 min. at 126 C.

Tensile strength, p. s. i 450 285 670 715 815 1, 825 1, 555 665 Thevarious parts and percentages expressed 40 REFERENCES CITED herein andin the appended claims are on a weight basis unless otherwise indicated.

The invention claimed is:

1. A rubber softening composition comprising from about 15 to 60%naphthenic acids, at least 15% of an aromatic petroleum derivative, andfrom 0 to of a neutral petroleum oil, said aromatic petroleum derivativehaving a viscosity of at least 10 cs. at 210 F., softening below 200 F.,and having a specific gravity ((14 of from 0.9 to 1.05.

2. The composition of claim 1 wherein the petroleum residue is anasphaltic material.

3. The composition of claim 1 wherein the petroleum residue is anextract obtained by treating petroleum with propane and phenol.

4. The composition of claim 1 wherein the petroleum residue is anEdleaneau extract.

5. A rubber softening composition comprising The following referencesare of record in the file of this patent:

UNITED STATES PATENTS Number

1. A RUBBER SOFTENING COMPOSITION COMPRISING FROM ABOUT 15 TO 60%NAPHTHENIC ACIDS, AT LEAST 15% OF AN AROMATIC PETROLEUM DERIVATIVE, ANDFROM 0 TO 50% OF A NEUTRAL PETROLEUM OIL, SAID AROMATIC PETROLEUMDERIVATIVE HAVING A VISCOSITY OF AT LEAST 10 CS. AT 210* F., SOFTENINGBELOW 200* F., AND HAVING A SPECIFIC GRAVITY (D420) OF FROM 0.9 TO 1.05.